FIG. 1 is a block diagram illustrating an example of one branch of a conventional broadband communications system, such as a two-way hybrid fiber/coaxial (HFC) network, that carries optical and electrical signals. Such a network may be used in a variety of systems, including, for example, cable television networks, voice delivery networks, and data delivery networks to name but a few. The communications system 100 includes headend equipment 105 for generating forward, or downstream, signals (e.g., voice, audio, video, or data signals) that are transmitted to subscriber equipment 145. Initially, the forward signals are transmitted via optical transmitters (not shown) as optical signals along a first communication medium 110, such as a fiber optic cable. In most networks, the first communication medium 110 is a long haul segment that carries light having a wavelength in the 1550 nanometer (nm) range. The first communication medium 110 carries the forward signal to hubs 115, which include equipment that transmits the optical signals over a second communication medium 120. In most networks, the second communication medium 120 is an optical fiber that is designed for shorter distances, and which carries light having a wavelength in the 1310 nm range.
From the hub 115, the signals are transmitted to an optical node 125 including an optical receiver and a reverse optical transmitter. The optical receiver converts the optical signals to radio frequency (RF), or electrical, signals. The electrical signals are then transmitted along a third communication medium 130, such as coaxial cable, and are amplified and split, as necessary, by one or more distribution amplifiers 135a–c positioned along the communication medium 130. Taps 140 further split the forward signals in order to provide signals to subscriber equipment 145, such as set-top terminals, computers, telephone handsets, modems, televisions, etc. It will be appreciated that only one branch of the network connecting the headend equipment 105 with the plurality of subscriber equipment 145 is shown for simplicity. However, those skilled in the art will appreciate that most networks include several different branches connecting the headend equipment 105 with several additional hubs 115, optical nodes 125, amplifiers 135a–c, and subscriber equipment 145.
In a two-way network, the subscriber equipment 145 generates reverse RF signals, which may be generated for a variety of purposes, including video signals, e-mail, web surfing, pay-per-view, video-on-demand, telephony, and administrative signals from the set-top terminal. These reverse RF signals are typically in the form of modulated RF carriers that are transmitted upstream through the reverse path to the headend equipment 105. The reverse electrical signals from various subscribers are combined via the taps 140 and passive electrical combiners (not shown) with other reverse signals from other subscriber equipment 145. The combined reverse electrical signals are amplified by one or more of the distribution amplifiers 135a–c and typically converted to optical signals by the reverse optical transmitter included in the optical node 125 before being provided to the headend equipment 105. It will be appreciated that in the electrical, or RF, portion of the network 100, the forward and reverse electrical signals are carried along the same coaxial cable 130. In contrast, the forward and reverse optical signals on the first and second communications media 110, 120 are usually carried on separate optical fibers.
It is well known in the art that spurious noise exists on the optical link connecting the optical transmitter and the optical receiver. More specifically, spurious noise is generated from the interaction of the laser included in the transmitter with the rest of the optical link. Spurious noise comprises very fast noise beats that rise out of the general noise floor and is typically viewed at the optical receiver output with test equipment, such as a spectrum analyzer. The beats are a product of Rayleigh backscatter in the optical fiber creating brief external cavities. Spurious noise beats tend to cluster at low frequencies, but they may also concentrate at higher frequencies. Spurious noise, therefore, affects both data signals and video signals.
The most effective method of preventing spurious noise is to optically isolate the laser, thereby preventing backscattered photons from impacting the laser. Disadvantageously, however, the cost of an isolator is expensive and sometimes exceeds that of the laser. Optical attenuation can also be used to reduce, but not eliminate, spurious noise, however, at the cost of reduced optical power. A 5 dB optical attenuator at the laser improves spurious performance by about 10dB. Another method to reduce the effect of spurious noise is to modulate the laser to increase its linewidth. Accordingly, the peak intensity of the central mode is reduced, thereby reducing the amplitude of the spurious noise beats. This is typically a more cost effective method.
FIG. 2 is a block diagram of a conventional optical transmitter including an oscillator for introducing dithering tones, which is a method of reducing spurious noise by continuously modulating the laser. In this example, three dithering tones are introduced in the optical transmitter 200 and generally appear below the RF frequency range that is designated for signal transmission. For example, dithering tones are introduced at, for example, three different frequencies via a 3-tone oscillator 205 typically within the range from 0 MHz to 5 MHz, where the reverse RF frequency range is from 5 MHz to 45 MHz. After filtering via a lowpass filter 210, the three tones are then combined with the main input RF signal. Typically, the composite power of the tones is set to a level that gives the best system performance at low channel loading since the laser is not being modulated as often due to the limited number of services and the infrequent transmission of signals. As a result, in-band intermodulation products are disadvantageously generated as the channel loading increases. Additionally, the composite power of the dithering tones reduces the system dynamic range available to desired signals. As a general rule, the intermodulation products should be at least 40 dB below a digital carrier signal and 57dB below a video carrier signal.
Dithering tones are not recommended, however, when video carrier signals are present because the dithering tones affect the viewing quality and signal processing of the video signals. Additionally, dithering tones affect a high channel loading system, such as a system transmitting all digital channels. What is needed, therefore, is an effective and efficient method and apparatus of mitigating the effects of spurious noise particularly when video signals and signals in a high channel loading system are transmitted.